Maximum payload capacity of the C-17 is 170,900 lb (77,500 kg), and its maximum gross takeoff weight is 585,000 lb (265,350 kg). With a payload of 160,000 lb (72,600 kg) and an initial cruise altitude of 28,000 ft (8,500 m), the C-17 has an unrefueled range of approximately 2,400 nautical miles (4,400 km) on the first 71 units, and 2,800 nautical miles (5,200 km) on all subsequent units, which are extended-range models with an additional fuel tank in the center wing box. These units are informally referred to by Boeing as the C-17 ER.[13] Its cruise speed is approximately 450 knots (833 km/h) (.74 Mach). The C-17 is designed to airdrop 102 paratroopers and equipment.